Ground target elimination



Jan. 31, 1950 R. F. MOZLEY GROUND TARGET ELIMINATION 7 Sheets-Sheet 1 Filed April 12, 1943 -INVENTOR ROBERT F. MOZLEY BY.

ui/fl. M.

ATTORNEY Jan. 31, 1950 MOZLEY 2,495,753

7 GROUND TARGET ELIMINATION Filed April 12, 1943 7 Sheets-Sheet 2 DRIVING 355-91 MOTORL v HOLLOW PIPE 37 CONNECTIONS 35 I c056 sin# 39 ALTIMETER AND MODULATOR r h smvcos 34 4s 47 I $ADIUS RECEIV T0 IME DELAY ER CONVERTER DEMODULATOR INDICATOR 49 5] 48 52 I L 45 L42 Trigger Gcie Pulse VJ IIIIIII IIIIII 28 2 2 as 95 Fa;-

T163 E4 INVENTOR ROBERT F. MOZL'EY ATTORNEY Jan. 31, 1950 R. F. MOZLEY GROUND TARGET ELIMINATION 7 Sheets-Sheet 3 Filed April 12, 1943 Trigger Pulse 83 cos"L INVENTOR I ROBERT F. MOZLEY fly/KM ATTORNEY 90 Dihedrcl l Angle Jan. 31, 1950 Filed April 12, 1943 R. F. MOZLEY GROUND TARGET ELIMINATION 7 Sheets-Sheet 4 l V I 1 l d I z v x x Di siorfing Amglifiers lol I a osuoouuvron 39 rcscV sec@ 5 DEMODULATOR MULTIPLIER DEMODULATOR sin! 'I: z J5.

INVENTOR ROBERT F. MOZLEY BY J M 2 ATTORNEY J 1950 R. F. MOZLEY 2,495,753

GROUND TARGET ELIMINATION Filed April 12, 1943 '7 sheets-sheet 5 DRIVING MOTOR AND HOLLOW PIPE CONNECTIONS 46 45 RECEIVER f 6 3 MOVING on: E GENERATOR 'fe N DICATOR 7 41 42 #3 is Trigger INVENTOR ROBERT F. MOZLEY BY fl/fl m ATTORNEY Jan. 31, 1950 R. FIMOZLEY 2,495,753

GROUND TARGET ELIMINATION N Filed April 12, 1943 7 Sheets-Sheet e I INVENTOR ROBERT F. MOZLEY ATTORNEY Jan. 31, 1950 R. F. MOZLEY 2,495,753

GROUND TARGET ELIMINATION Filed April 12, 1943 7 Sheets-Sheet 7 v kIQS TronsmiHer Trigger .A

99 Trunsmiited Pulse 20| Wide em 22; M G 86 v9. 88 I ovlng qe 0 2 r203 209- Firs! Range Gufe zlwv 4 Second Range Gcfe Ground Signal 206 lnpu'r #0 Peak Volfmeiers 207 Pec'lk Volfnqe Oufpuf 1 INVENTOR fr-" ROBERT F. MQZLEY ATTORNEY Patented Jan. 31', 1950 GROUND TARGET ELIMINATION Robert F. Mozley, Hempstead, N. Y., asslgnor to The Sperry Corporation, a corporation of Delaware Application April 12, 1943, Serial No. 482,807

28 Claims.

My "invention relates to aeronautical devices and electronic circuits, and concerns particularly arrangements for measuring distances to ground points from an aircraft. I

An object of my invention is to provide methods and apparatus for increasing the safety of aerial transportation and aerial navigation.

Another object is to provide methods and apparatus for eflecting traffic control, particularly runway approach control at crowded airports.

A further object is to provide methods and apparatus'for making safe landings at regular landing fields as well as at emergency locations under conditions of poor visibility or when making blind landings for other reasons.

It is likewise an object to provide improved arrangement for preventing collisions in the air with other aircraft, particularly when flying low, as well as for preventing collisions with stationary obstructions such as sharp cliffs, water towers, tall chimneys and the like.

Moreover, it is an object of the invention to increase the effectiveness of radio-type object or target locators in general, including target locators for patrol and fire control purposes as well as for other purposes.

In addition it is an object of my invention to provide a method of aircraft runway approach whereby the area ahead of an aircraft is swept with a radio beam, and reflected beams are detected, except that beams reflected from the ground surface are eliminated, so that the detection of a reflected beam indicates the presence of an obstacle and warns a pilot to postpone landing or to change the angle of runway approach accordingly.

It is also an object of my invention to provide methods and apparatus for elimination of ground reflections in apparatus for locating and tracking moving objects, particularly from an aircraft.

A further object of my invention is to provide arrangements for computing distance to ground along an angle from a point in the air.

An additional object of my invention is to provide arrangements for electrically measuring distance to ground in any direction.

still another object of my invention is to provide arrangements for controlling a receiver, such as the receiver of an object locator, to make the receiver unresponsive to ground reflection.

Stil1 another object is to provide arrangements 2 for varying the effective range of an object locator in accordance with variations in distance to ground. An additional object of my invention is to pro vide automatic range measuring arrangements. It is also an object of my invention to provide arrangements for compensating for the effect of pitch and roll of an aircraft in the computation of distance to ground.

It is another object of my invention to arrange dynamo electric apparatus for computing products of sinusoidal functions or products of functions of variable angles.

It is also an object of my invention to provide arrangements for simulating cosecant and secant curves in response to sine and cosine input indications.

It is also an object of my invention to provide arrangements for converting magnitude of an electrical quantity, such as voltage, into a time delay proportional to the magnitude of the electrical quantity.

Other and further objects and. advantages will become apparent as the description proceeds.

In certain types of radio locating or searching systems, what is known as type-C indication is employed. This signifies that a target or object which is to be detected from an aircraft, for example, is located by scanning with a radio beam which is caused to traverse a spiral course. In such apparatus an indicating device, such as a cathode ray oscilloscope, is provided in which a cathode ray beam is also caused to move spirally and to produce an indication on the screen of the cathode ray tube in case a target is intercepted by the scanning radio beam. This type of apparatus has the merit that the position of the target in both azimuth and elevation is indicated upon a single screen.

However, if the aircraft carrying the locating equipment is flying low or the distance range of the locator is greater than the altitude of the aircraft, the scanning radio beam will thus intersect the ground for a considerable solid angle of the angular range of the cathode ray oscilloscope indicator. Reflections from the ground will take place throughout the area of the screen corresponding to the portion of the solid angle of the scanning system subtended by the ground plane. In the event that a target should come in between the aircraft carrying the locating equipment and ground, such a target is liable to be obliterated by the ground reflections.

It is accordingly an object of my invention to provide methods and apparatus for enabling targets to be located anywhere within the angular range of locating equipment, regardless of the height at which the searching aircraft is flying or the relationship of the target to the ground plane.

Other and further objects will become apparent as the description proceeds.

In carrying out my invention in its preferred. forms, I provide attachments for radio beam scanning apparatus for ascertaining the distance to ground and controlling the receiver of the scanning apparatus to eliminate ground reflection whenever the scanner is so oriented that the distance to ground along the radio beam is less than the normal linear or radial distance range of the apparatus.

In accordance with one embodiment of my invention, the distance to ground along the radio beam axis at any instant is computed electrically from the altitude of the aircraft and the instantaneous angles of the radio beam of the searching apparatus. Mechanism is provided for reducing the radial range of the locator to a distance slightly less than the computed distance to ground. For example, means may be provided for producing a square wave voltage having a time duration varying in accordance with the computed distance to ground. The indicator is provided with a voltage control for making the indicator effective only when the control is energized. The square wave, having a time duration corresponding to computed distance to ground, is applied to the voltage control of the indicator so that when the searching beam is directed toward the ground, the indicator remains efi'ective only for a duration of time less than that required for a transmitted signal to travel a distance equalling the distance to ground along the radio beam axis and return along the same path to the radio locator. In this manner, any target between the aircraft and ground produces an indication, but the ground produces no indication or reflection.

In accordance with another embodiment of my invention, the distance to ground is directly measured electrically independently of any indications of altitude or angle. For this purpose I may utilize a moving gate electronic servo circuit which produces a. voltage having a time delay corresponding to the distance to ground along the radio beam.

A better understanding of the invention will be afforded by the following detailed description considered in connection with the accompanying drawings, and those features of the invention which are believed to be novel and patentable will be pointed out in the claims appended hereto.

In the drawings,

Fig. 1 is a view of the screen of an indicator for a type of radio locator system in which my invention may be employed.

Fig. 2 is a perspective view of the ground plane and of an aircraft shown within a hypothetical hemisphere having a radius equalling the maximum range of a radio locator installed on the aircraft, the sphere being cut by the ground plane.

Fig. 3 is a schematic diagram of a radio locator receiving system employing one embodiment of my invention, in which ground distance is automatically computed.

Fig. 4 is a schematic diagram of one form of 4 remote angle indicating system which maybe employed in connection with the apparatus of Fig. 3.

Fig. v5 is a schematic diagram of a modification in the arrangements illustrated in Figs. 3 and 4 which may be employed for electrical multiplication of angular indications.

Fig. 6 is a circuit diagram of a device for producing a time delay or gate length proportional to a voltage, which is in turn proportional to the distance to ground along the axis of the scanner of a radio locator. t

Fig. 7 is a graph representing the gate produced by the apparatus of Fig. 6.

15 Figs. 8 to 12, inclusive, are perspective views of three dimensional diagrams, Fig. 8 being a diagram showing a ground plane in relation to a system of rectilinear coordinates, Figs. 9 and 10 being diagrams of spherical pyramids drawn to illustrate the manner of mathematically deter- 'mining the relationship between the rectilinear coordinates of Fig. 8 and spherical coordinates, Fig. 11 being a diagram in which the representations of Figs. 8, 9 and 10 have been simplified and combined to form a single diagram and Fig. 12 being a diagram illustrating the conversion of rectilinear to spherical coordinates.

Fig. 13 is a circuit diagram of a modification in aportion of the apparatus of Fig. 3.

Fig. 14 is a graph of a secant curve representing one factor of a computation used in the apparatus of Fig. 3.

Fig. 15 is a graph of an electrical function simulating the curve of Fig. 14.

Fig. 16 is a schematic diagram of a radio locaxtor receiving system corresponding to the apparatus of Fig. 3 but employing automatic electrical means of measuring distance to ground instead of employing means for computing such a distance to ground.

Fig. 17 is a schematic diagram illustrating the principle involved in electrically automatically changing the range in accordance with distance to ground.

Fig. 18 is a circuit diagram of a portion of the apparatus of Fig. 16, more particularly the apparatus for auto ranging, and

Figs. 19A to 19H, inclusive, are graphs explanatory of the principles of operation of the apparatus of Figs. 16 and 18.

Like reference characters are utilized throughout the drawings to designate like parts.

General explanation of illustrative locator system 5 For the sake of illustration, I shall describe a manner of carrying out my invention in connection with a spiral-sweep pulsed-microwave radiobeam scanner of the type represented schematically in Figs. 3 and 16. In this type of radio 10- c0 cator or searching scanner there is a radiator H comprising a parabolic reflector [2 with a parabola axis I3, and having a dipole antenna (not visible in the drawings) mounted at the focus of the parabola I2. The radiator ll serves both for 65 the purpose of projecting a radio beam in pulses of microwave oscillations, and for the reception of any pulses which may be reflected in case the beam is intercepted by a target or an obstacle to aviation. The parabola axis i3 is also the radio 70 beam axis.

Such a system is illustrated and described in greater detail in the copending application Serial No. 441,188, filed April 30, 1942 by White, Holschuh, Mieher and Shepherd. It will be un- 75 derstood, however, that my invention is not 11mited to use with a spiral spinner scanner or with the specific type of radio locator described in said application, and schematically represented by drawings and the description of the present application.

In this type of radio locator the radiator II is so mounted as to be rotatable about a spin axis, represented in Fig. 3 by a supporting shaft I, so that the beam 13 tends to describe a cone. The radiator is also pivoted about an axis l5 transverse to the spin axis l4 and which rotates with the shaft ll. The pivot axis l5 may be referred to as a nod axis. Nod motion of the radio beam 13 causes the apex angle of the cone described thereby to change progressively to produce a spiral sweep. For pivotally supporting the radiator II, a yoke i5 may be mounted on the shaft l4 and the radiator parabola l2 may be mounted on a T member I! which has trunnions supported by the yoke I6. The members l4, l6 and H are in the form of hollow pipes acting as a jointed waveguide type of transmission line for transmitting microwave radio energy to and from the radiator The radio scanner, together with associated apparatus which will be described more in detail hereinafter, is mounted on an airplane l8, shown in Fig. 2. In Fig. 2 there is shown a hemisphere IQ described by a radius of length R, which is the maximum radial range of the radio scanner. Since it is impracticable for the nod motion or rotation of the radiator l I about the nod axis R5 to be made greater than 90 in most installations, the maximum solid angle which may be assumed to be scanned by the radiator l l is the hemisphere is shown in Fig. 2. Means are provided for rotating the shaft it carrying the radiator II to produce a spinning motion, and means (not shown) are also provided for rotating the radiator I i about the axis Hi to produce a nod motion. The nod motion is made relatively slow in comparison with the spin motion so that the radio beam axis l3 describes a spiral on the surface of the hemisphere l9. The projection of this spiral trace on a plane circle is represented in Fig. 1, which also represents the trace of the cathode ray beam of an indicator which is used in connection with the radio locator apparatus. Such an indicator may comprise a cathode ray tube having a screen 2i. The spiral trace is represented by the curve 22 in Fig. 1.

In radio locator apparatus of the spiral sweep type, means are ordinarily provided for preventing the cathode ray beam of the indicator from striking the screen 2i unless a target is intercepted by the radio beam travelling along the axis l3. A spiral trace 22, shown in Fig. 1, therefore does not actually appear on the screen 2! under normal conditions. However, in the event of interception of the beam by a target, such as another airplane, or a sea-going vessel, or some other obstacle to aviation, such as a tower or a mountain peak, a bright spot 23 appears on the screen 2! and the angular position of the spot 23, as well as its radial distance from the center 24 of the screen 2|, serves as an indication of the position of the target in azimuth and elevation with respect to the airplane l8. It will be understood that the screen 2I is provided with suitable calibrations so as to relate the indications thereon to the azimuth and elevation angles of the radio beam axis l3 intercepted by a target 23' (Fig. 2).

In the event that the altitude h at which the plane i8 is flying is less than the radio range R of the radio locator equipment, the hemisphere IE will have the lower portion thereof intersected by a plane 25, representing the ground plane or surface of the ground. Under this condition the lower segment 26 of the indicator screen 2i, corresponding to the solid angle subtended by ground plane 25, will produce reflections for any position of the radio beam axis i3 in which the beam axis l3 strikes the ground. Accordingly, the segment 26 of the indicator screen 2! will apparently be covered with target indications, which may be referred to as ground targets or ground reflections. In the event that some obstacle'to aviation or a hostile airplane should be present along a line drawn from the airplane 18 to a point 20 on the ground, the indication of such a target on the indicator screen 2i is liable to be obliterated by the mass of ground reflections 28.

For the pupose of overcoming difliculty from ground reflections, I eliminate such reflections from the screen 2| by reducing the radial distance range of the radio locator equipment from the maximum value R. to shorter range values while the radiator l l is in such angular positions that the radio beam axis l3 strikes the ground.

The problem of eliminating surface reflections arises when an aircraft is flying over bodies of water as well as when flying over land masses, an I employ the term ground in the description and claims to refer to the surface of such bodies of water as well as to the earth's land surface.

Computer type ground reflection eliminator For eliminating ground reflections in the type of apparatus illustrated in Fig. 3, the radial distance r to ground is computed from the angular positions of the radiator II and the altitude of the airplane, and the range of the radio locator apparatus is automatically reduced in accordance with the computed distance to ground. In order to obtain continuous indications of the angular positions of the radiator I l for the computation of radial distance to ground, suitable angular position indicators are provided. Preferably remote position indicators are employed in order to simplify the construction. For example, I may utilize a transmitter 26 of the type used in conventional electrical angle transmission systems, and arranged for transmitting an electrical indication instead of a mechanical indication to a distance.

The transmitter 26 is provided with a stator 21 and a rotor (not visible in Fig. 3) secured to a shaft 28, which in turn is mechanically connected by gearing 29 to the spin axis driving shaft I 4. For reasons which will be explained hereinafter, the stator 27 may also be so mounted as to be adjustable in angular position.

For transmitting indications of nod angle, a similar angle transmitting system may be provided comprising a transmitter 3| with a stator 32 having a rotor (not visible in Fig. 3) secured to a shaft 33 which is secured to the member I! directly supporting the radiator ll. As in the case of the stator 21, the stator 32 may also be so mounted as to be adjustable in angular position, for reasons which will be explained hereinafter.

Suitable means are provided for continuously computing the radial distance to ground in terms of altitude and the angular indications provided by the transmitters 26 and 3|. For example, an electric computer 34 may be provided having one input connection in the form of a pair of electrical conductors 35 from the spin angle transmitter 26, and a second input connection in the form of a pair of electrical conductors 31 from the nod engle transmitter 3|. For indicating altitude, suitable apparatus represented by a rectangle 38 may be provided for producing an electrical indication proportional to altitude. Apparatus suitable for this purpose is shown and described in U. S. Patent Re. 21,955, Novem ber 25, 1941, to J. G. Chafiee. The apparatus 38 has an output connection through a pair of conductors 39 which serve as a third input connection for th computer 34.

It will be understood that microwave pulse radio locator apparatus of the type illustrated includes a radio receiver 4| and an indicator 42 having the screen 2| on which the indications of the presence of a target appear. Such a receiver has an input connection, ordinarily in the form of a rectangular hollow pipe wave-guide 43, leading from the radiator II. In the schematic drawing there is shown a box 44 which is assumed to contain a driving motor for rotating the shaft l4, as well as suitable hollow pipe connections for transmitting to the pipe 43 radio frequency energy received through the hollow shaft l4.

In such systems also a suitable transmitter (not visible in the drawing) is provided for transmitting pulses from the radiator II and associated with the apparatus there is also a trigger pulse source (not shown) for synchronizing the transmitter and the receiver 4|. Such a trigger pulse source is arranged for synchronizing a wide gate supplied to a receiver connection 45 after the termination of the transmitted pulses, for rendering the receiver responsive only to refiected signals. A suitable connection 46 is provided from the receiver 4| to the indicator 42 for producing indications when reflections are received by the receiver 4 The indicator 42, however, is also provided with a control connection, represented by a pair of conductors 41, for effectively reducing the range of the target responsive apparatus comprising the receiver 4| and the indicator 42 in accordance with signals supplied to the control connection 41. Although the apparatus is shown as having the control connection 41 applied to the indicator 42, it will be understood that the arrangement may also be such that that control connection is applied to the receiver 4|, or to some other suitable portion of the apparatus.

In the specific embodiment of the apparatus illustrated, the effective distance range of thevoltage gate or square wave which is applied to the input connection 41 of the indicator 42. Such means may take the form of an electronic circuit designated as a radius-to-time delay converter, represented in Fig. 3 by a rectangle 48. For reasons which will be explained hereinafter, the output indication of the computer 34 in the embodiment of Fig. 3 is in the form of a modulated alternating current or voltage having a peak amplitude varying in accordance with the radial distance to ground r. Under such conditions, a demodulator 49 may be interposed between the computer 34 and the radius-to-timedelay converter 48, for producing a radius voltage varying as the amplitudes of the voltage peaks in the alternating output of the computer 34.

AS shown, there is a pair of conductors 50 serving as an output connection from the computer 34 and an input connection to the demodulator 49, and there is a pair of conductors 52 serving as an output connection from the demodulator 49 and an input connection to the radius-to-time delay converter 48. The device 48 may also have a trigger pulse connection represented by a pair of conductors 5| for synchronizing it with the pulse transmitter (not shown) which also synchronizes the wide gate of the receiver 4|.

Although my invention is not limited to a Darticular method of mounting the radio locator apparatus, the electrical computation of radial distance to ground ma be simplified, and accordingly a simpler form of electrical computer 34 may be employed, if the equipment is so mounted, or-the aircraft carrying it is so flown, that the shaft |4 remains horizontal, and some reference axis transverse to the shaft |4 also remains horizontal. For example, the equipment may be mounted on a platform maintained horizontal by suitable gyroscopic controls, or the radiator may be mounted in the nose of an aircraft with the axis of the shaft |4 along the fore and aft line of the fuselage, and the aircraft may be flown level whenever the indicator 42 is to be observed. However, to avoid necessity for maintaining the apparatus in a predetermined level plane, whether it is mounted directly on the aircraft or on a platform movable with respect to the aircraft, I may also provide suitable means for compensation of variations in the reference points for spin and nod angle from fixed references which would exist for level flying conditions. To this end, the stator 21 of the spin angle transmitter 26 is made adjustable in angular position, and, likewise, the stator 32 of the nod angle transmitter 3|.

For adjusting the angular positions of the stators 21 and 32 to predetermined reference positions, a yro system 53 maybe provided comprising a. schematically represented gyroscope 54 with a gimbal shaft 55 parallel to the axis of the spinner driving shaft l4, and a perpendicular shaft 56 which remains horizontal by virtue of the action of the gyroscope 54, which maintains its axis vertical. provided for adjusting the angular positions of the stators 32 and 21 in accordance with relative angular positions of the gyroscope shafts 55 and 56.

For adjusting the angular position of the stator 21, a self-synchronous angular transmission system, such as a Selsyn system for example, may be provided comprising a Selsyn transmitter 51 and a Selsyn receiver 58 with polyphase conductors 59 joining the transmitter and receiver 51 and 58. It will be understood that the transmitter 51 has a rotor secured to the gyro fore and aft shaft 55, and the receiver 58 has a rotor mechanically connected through gear 6| to the spin axis angle transmitter stator 26 for adjustil'lg the position of the stator 26 to compensate for roll of the aircraft or of the platform on which the radio locator may be mounted. It will also be understood that such self-synchronous systems are provided with a source of single-phase exciting current 62 through pairs of conductors 63 leading to both the transmitter and receiver.

A similar self-synchronous transmission system for compensating the angular indication of the nod axis is provided comprising a Selsyn" transmitter 64 with a rotor secured to the gyroscope axis 56, a receiver 65 with a rotor mechanically connected by gearing 68 to the angular Angular transmission systems are transmitter stator 32, and conductors 61 joinin tate the use of slip rings, or the like (not shown) for carrying the electrical connections through the conductors 31, 61, etc. It will be understood that in practice the transmitter 3| may be connected to the means (not shown) for rotating the radiator ll about the nod axis IS.

The transmitters 26 and 3| may, if desired, be of the type sold as Telegon transmitters, represented schematically in Fig. 4. In such apparatus there is a stationary exciting winding 68 arranged for magnetizing an axially extending magnetic rotor 69 with transverse projections II. There is a stator comprising a pair of crossed pick-up windings I2 and 13 arranged at right angles to one another and having magnetic axes perpendicular to each other and to the axis of the rotor shaft 28. For use in my apparatus, the windings 12 and 13 are mounted on the angularly adjustable stator frame 21 which may have a shaft 14 securedthereto connected to the gearing SI of Fig. 3.

For use with the type of computer 34, described for the sake of illustration in the present application, only one of the stator windings I2 and 13 need be employed, or, if desired, the stator windings l2 and 13 may be connected in series to the output conductors 35 Or 31. Alternating voltage having the same frequency as the source supplied to the exciting winding 68 is induced in the pick-up windings I2 and 13. The arrangement is such that as the rotor 69 rotates with the shaft 28, the alternating voltages vary in peak value or are modulated, as the sine or cosine of the angle, according to the reference point taken.

The apparatus 38 may comprise a known form of altimeter, such as that shown and described in the above mentioned Reissue Patent 21,955, measuring vertical distance to ground (not shown), with a source of alternating current and means driven by the altimeter for varying the amplitude of the alternating current in accordance with the altimeter indication. In this manher, the device 38 produces a modulated alternating voltage fluctuating in peak value in accordance with the altitude indication. The altitude indication is therefore of the same type as that provided by the transmitters 26 and 3| The computer 34 may be of the type described in the copending application of Herbert Harris, Jr., Serial No. 474,052, filed January 28, 1943, in which an output is produced proportional to a product or quotient of the amplitudes or envelopes of several modulated input alternating currents or voltages.

The radius-to-time delay converter 48 may comprise a pair of electric valves, such as vacuum tubes, connected in cascade, as illustrated in Fig. 6. As illustrated, it comprises a pair of triodes 16 and Ti having load resistors 78 and '19, respectively, and a common cathode resistor 88 with a common source of anode current 8| The triode 16 has a control electrode or grid 82 coupled through a coupling condenser 83 to the trigel pulse input terminals and connected I through a grid resistor 84 to the radius voltage input terminals 52. The tube 11 has a control electrode 85 capacity coupled from the anode of the tube I6 by a. condenser 85' and positively biased by means of a resistor 86 connected to a point at positive potential, for example, to the positive terminal of the anode supply source 8|. For adjusting the tubes to the proper portions of their characteristic curves a bias source 15 may be provided for the tube 18.

The circuit of Fig. 6 is designed to produce a square wave 81, as illustrated in Fig. 7, having a fixed amplitude and having a time duration t dependent upon the voltage applied between the radius voltage input terminals 52. In Fig. 7 the voltage amplitude of the square wave is plotted vertically and time is plotted in a horizontal direction.

Since the control electrode 85 of the tube 11 is positively biased, the tube 11 normally conducts current, a large voltage drop takes place in the load resistor I9, and the voltage between the gate output terminals 41 is a minimum, as represented by the portion 88 of the graph of Fig. 7. When the circuit is triggered by a trigger pulse applied to the terminals 5 I, which also triggers the transmitter (not shown) and the delayed wide gate 45 of the receiver 4|, the tube I6 becomes conducting driving the control electrode 85 of the tube 11 negative, so that the voltage output of the tube 11 appearing between the gate terminals 41 abruptlly rises along the line 89 in the graph of Fig.

The tube 15 remains conducting for a period of time depending upon the magnitude of the radius voltage supplied between the terminals 52. Thus, the voltage applied at the radius voltage terninals 52 serves as a variable bias for the tube 1 After the trigger pulse has died away the current flow through the tube 16 is determined by the grid bias, which in turn depends upon the magnitude of the voltage applied at the radius terminals 52. This current flows through the cathode resistor 88 and thereby controls the oathode bias of the tube 11. The greater the voltage at the terminals 52, the greater the potential of the cathode of the tube 11 and therefore the greater the length of time required for the coupling condenser 85' to discharge sufilciently for the potential of the control electrode 85 to rise to cut-off and again render the tube 11 conducting. When the tube 11 becomes conducting, the output voltage across the gate terminals 41 again falls along the line 9! to the minimum value of the graph of Fig. 7. As represented by the double arrow 92 in Fig. '7, the portion 9| 0f the graph may be moved to the right or to the left by varying the magnitude of the voltage suppliedbetween the radius terminals 52.

Since the computer 34 continuously computes the radial distance to ground, that is, the distance along the radio beam axis l3, the time duration of the portion 81 of the graph in Fig. 7 represents the distance to ground. The indicator 42 has a control gate applied to its control terminals 41 which permits the indicator to be effective only for a time duration fixed by the length of the control gate. The adjustment of the receiver 4| and the indicator 42 is such that the time required for a signal to be transmitted the distance represented by the length of the control gate 81 is slightly greater than the actual time duration of the gate 81.

Consequently, if any target or obstacle should 11 appear at a point 93, Fig. 2, between ground and the airplane IS, an indication thereof will appear upon the screen 2| of the indicator 42. However, after the time interval required for a transmitted Operation in general The operation of the apparatus of Fig. 3 as a whole is briefly as follows when level flight is assumed. With respect to the aircraft supporting the radio locator, the radiator or scanner ll executes a spiral motion causing the beam 13 to sweep a solid angle of space which may be a complete hemisphere. If any target intercepts the beam l3, a bright spot 23 appears on the screen 2| (Fig. 1) in a position corresponding to that of the target. sponsive to spin angle, nod angle, and altitude continuously computes the distance to ground along the beam I3 and.reduces the radial distance range of the apparatus to a value just under the radial distance to ground whenever the beam l3 points toward the ground plane 25 (Fig. 2). In this manner ground reflections are avoided. For the case when the spinner shaft i4 is horizontal and there is no pitch or roll of the support for the scanner, the computer 34 may be arranged to solve the equation:

l sin [1 cos where r is the radius or length of the beam I 3 from the scanner ii to the intersection of the beam with ground plane 25, p is the nod angle,

and 0 is the spin angle. The nod angle t is measured from the spin axis, the axis of the spin shaft i4, and the spin angle 0 is measured from a reference point vertically below the shaft H, which is assumed to be horizontal for this case. The angle 1/1 never exceeds 90 in practice and never becomes negative, so that sin 0 never becomes negative. When the beam i3 points above the horizon. so that it cannot strike the ground plane the angle 0 lies between 90 and 270 and cos 6 is negative, making the computed value of r theoretically negative. However, by means which will be described hereinafter, the cosine factor may be prevented from becoming negative and may be caused to assume a suitable constant value ning shaft l4, the effect of roll is fully compensated by correction of the spin angle 0 by adjustment of the stator 21, by means of the gyroscope system 53. Provided pitch does not become excessive, it is substantially compensated by correction of the nod angle p by adjustment of the stator 32 by means of the gyroscope system 53.

In case the spinner shaft I4 is not assumed to be normally horizontal, a more complex solution for r is required and the principle involved may However, the computer 34, re-

cabana best be understood by a mathematical analysis.

For the sake of simplifying the construction, I have referred to the possibility of making a certain simplification in the computation of radial distance to ground, and I have referred to the possibility of mounting the apparatus in a particular manner with respect to the aircraft to simplify the computation. However, my invention is not limited to the mounting arrangement specifically suggested. The principle involved in.

Computation of distance to ground For the general case, the ground plane may be assumed to be oblique with respect to any set of coordinates. The principle involved will be better understood by considering the equation of the ground plane 25 in spherical coordinates about P as an origin. The equation of the ground plane in Cartesian or rectilinear coordinates is:

where h is the altitude of aircraft and therefore the length of a normal PG from the origin P to the ground plane 25, and L, M and N are the direction cosines of the normal PG with respect to-the X'X, YY and Z'Z axes shown in Fig. 8.

The projection of the line PG on the XY plane is along a'line PA shown in Fig. 11. A plane through PAG includes the Z'Z axis and is perpendicular to the XY plane.

Referring to Fig. 9, and applying the'theorems of spherical trigonometry, since the dihedral angle XAG is a right angle,

cos a=cos n cos b (2) where a=the angle XPG g=the angle X'PA b=the angle GPA Referring to Fig. 10, since the dihedral angle YAG is a right angle,

Referring to Fig. 12 and converting to spherical coordinates:

:r=r cos cos a (8) 11=r cos sin 0 (9) z=r sin (10) by substituting values from Equations 5 to 10 in the equations for the ground plane, Equation 1,

13 the ground plane equation in spherical coordinates is obtained:

If the Z'Z axis is assumed to be the spin axis through the shaft 4 of the scanner, the spin angle is and the nod angle ,0 (measured from the Z'Z axis instead of from the XY plane) is 90. Equation 11 may then be rewritten computer 34 may be supplied with angular inputs from the gyro transmitters 64 and 51 as well as the spin and nod angle transmitters 26 and 3|, and the altimeter 38. In this case the computer 34 is designed to solve Equation 12 above.

It is unnecessary, however, for the computer 34 i to be capable of computing values of r greater than R, the maximum possible range of the radio locator equipment.

Simplified computation If the aircraft is assumed to be so flown that the spin axis remains parallel to the ground, which is usually the case for gun director systems, the angle b is zero. If the aircraft is flown without either pitch or roll, in general, or if the spin axis is parallel to the fore and aft line of the aircraft, and the aircraft if flown without pitch, the angle 9 is a constant, and becomes zero if the XX axis is assumed to be vertical.

With this simplification, Equation 12 becomes h 7 sin p s 0 (13) This simplifies the apparatus required to form the computer 34.

If the spin axis of the scanner is parallel to the fore and aft line of the aircraft, roll of the craft is fully compensated, by correction of the spin angle 0, as shown in Fig. 3, by means of the gyro transmitter 5! for adjusting the stator of the spin angle transmitter 26. Likewise, if the pitch of the aircraft does not become excessive any error due thereto in employing the simpler equation, 13, is substantially compensated by modification of the indicated nod angle p by means of the gyro transmitter 64, which adjusts the angular position of stator of the nod angle transmitter 3|.

Modified computation The electric computer 34 has been represented as being so designed as to divide a variable voltage by the product of two other variable voltages.

In order to obviate the necessity for utilizing a For ex- 7 cos ;b cos y cos cos 6 cos b sing cos 1; 5 n 0-sin b sin 5 Ill cos b cos g sin 0 cos 0 +cos b sin g sin ill sin 0- sin b cos t connection. One of the transmitters, for example the transmitter 3|, may have a. rotor magnetizin winding 96 energized by the source of exciting current 63, and a pick-oil. winding 66. It will be understood that the pick-ofl winding 96 is connected by means of a shaft 91 or the like to the gearing 66, shown in Fig. 3, to maintain the stator in the proper angular adjustment with respect to the horizontal.

The other angle transmitter, in this case the transmitter 26, has its rotor exciting winding 68 connected to the pick-ofi winding 96 of the transmitter 3|. The rotor of the transmitter 3| rotates with the shaft 33 according to the nod angle of the radiator II and the rotor 69 of the transmitter 26 is driven by the shaft 28 according to the spin angle of the radiator II. The voltage induced in the pick-off winding 96 has the frequency of the excitation source 63 and is proportional to the product of the voltage of the input source 63, which is constant, and the sine of the angular position of the rotor shaft 33.

A sinusoidally modulated current is utilized for magnetizing the rotor 69 of the transmitter 26. Accordingly, a voltage is induced in its pick-oil. winding 12 which has the frequency of the excitation source 63, but has a peak value or amplitude proportional to the product of the sine of the angular position of the shaft 33 and the cosine of the angular position of the shaft 28.

It will be understood that the angular reference points are so chosen that the function is the sine function in one case and the cosine function in the other case.

The voltage appearing in the pick-off windings 12 may be supplied to a suitable quotient computer as a divisor. It will be understood that a voltage modulated in accordance with the altitude will also be supplied to the computer so that the result is the quotient of the altitude voltage and thevoltage supplied by the cascaded transmitters 3| and 26 of Fig. 5.v

If desired, necessity for the use of a quotient computer may be eliminated by converting the voltages supplied by the transmitters 3| and 26 into a quantity varying as the reciprocal of the product, or two separate voltages proportional to the cosecant and the secant of the nod angle and the spin angle, respectively, may be produced.

For the purpose of not only producing such a reciprocal voltage or voltages, but also producing a voltage or voltages of constant maximum amplitude for the angles'which would have negative values of the secant or the consecant. I may utilize a distorted amplifier circuit such as shown in Fig. 13, for example.

In the arrangement of Fig. 13, demodulators ||i|, I02 and I63 are provided for converting the modulat ons of the carrier voltages between the conductor pairs 31, and 39, respectively. into fluctuating voltages representing the envelopes with the carrier (excitation) frequencies removed. For converting the output voltages from demodulators mi and M3 into voltages representing the secant and the cosecant. respectively, distorting amplifiers I04 and I65 are provided. These may be electric valves of the vacuum tube type, for example, such as pentodes having control electrode or grid circuits connected to the outputs of 15 the demodulator-s IOI and I03, respectively, and having load circuits with output connections I and I01, respectively, serving as input connections to a schematically represented multiplier circuit I08. It will be understood that such pentodes also have screen grids and suppressor grids.

The altitude demodulator I02 may have an out put connection comprising a pair of conductors I09 serving as an input connection to the multiplying circuit I08, which may therefore be arranged to multiply directly three fluctuating unidirectional voltages supplied by the connections I00, I01 and I09 to give a unidirectional output voltage. Such an output voltage is supplied to a pair of conductors 52 serving as an input connection to the radius-to-time delay converter 48 shown in Fig. 3.

The distorting amplifiers I04 and I05 are similar in arrangement and principle of operation and therefore only one of them need be described in detail. The amplifier I04 has a load resistor III connection in series with its anode lead to the positive terminal of an anode supply source H2. The anode voltage is supplied to the multiplying circuit I08 through the connection I06, and the output of the demodulator IOI is applied to the control electrode through a pair of conductors I I3. If necessary, the demodulator IOI may include an amplifier for supplying to the tube I04 a voltage of sufficient magnitude to enable the amplifier I04 to distort the input voltage. For biasing the tube I04 at the point at which increasing values of input voltage produce successively greater degrees of saturation, a bias voltage source II4 of suitable voltage is provided.

Referring to Fig. 14, representing a secant curve plotted against angle, it will be observed that the portions of the curve in a region near zero and 180 are relatively flat as compared with a cosine curve which has a relatively sharp peak. Likewise, the portions of the curve in the region near 90 and 2'70 degrees are very steep as compared with the slope of a cosine curve near 90 and 270 degrees. Furthermore, the slopes at all points are reversed as compared with the slopes of a cosine curve. The reversal of slope is produced by a vacuum tube amplifier which is resistance coupled to the output because such an amplifier acts as a phase inverter.

The flattening of the curve near zero degrees and the steepening of the slope near 90 degrees and 270 degrees is accomplished by the distorting characteristic of the amplifier I04.

1 Although my invention is not limited to the use of any particular class of electric valves or vacuum tubes, the required characteristics may be obtained by the use of tubes having relatively sharp cut-off and having their plate voltage versus plate curves crowded more closely together to the zero by its curve. For example, pentode vacuum tubes may be used of the 6AC7 or the 6SJ'Z type.

When utilizing tubes of this type referring to the tube I04 shown in Fig. 13, the load resistor III preferably has a resistance greater than that which provides the greatest degree of linearity of the load characteristic, for example, two or three times the resistance for maximum linearity. In order to obtain the desired distorting characteristics for increasing voltage inputs it is desirable to have the load line of the tube, drawn on the plate current-plate voltage curve, less in slope than the load line which provides maximum linearity. This is accomplished by utilizing relatively high load resistance.

It will be recalled that when the radiator II, with reference to Fig. 3, is in the angular positions between 90 and 2'70 degrees, the radio balm I3 is pointing upwards, and the computed values of distance to ground have no significance because the beam I3 never strikes the ground. The distorting amplifier I04 provides a constant output voltage for the values of 0 between 90 and 270 degrees because during these angles the value of cosine becomes negative so that the control electrode of the tube I04 is simply driven more negative. The value of the bias source H4 is such that the tube I04 is biased slightly beyond cut-off and therefore the output voltage for zero input and slightly higher is equal to the voltage of the supply source II2. Accordingly, negative input voltages have no further effect and'between 90 and 270 degrees the output voltage remains as a constant value, the voltage of the source III as represented by the line I I5 in Fig. 15.

Inasmuch as the secant curve in Fig. 14 becomes infinite in value in the regions near 90 and 270 degrees, corresponding to values of the cosine near zero, such values are beyond the range of the amplifier I04/and its output voltage actually reaches the maximum voltage, that of sourse III over a wider angular range, e. g., between approximately and 2'75 degrees, as indicated in Fig. 15.

The constants of the circuit may be so chosen that the maximum voltage II5 of the output curve shown in Fig. 15 is approximately equal to the radius voltage corresponding to the maximum range R of the locator apparatus. In this mannerv the computer serves to supply a control voltage to the indicator 42 which reduces the effective range of the receiver and indicator when the radio beam I3 is pointed toward the ground, but has no effect when the radio beam is pointed upwards, or makes such a small angle with the ground that the distance to ground exceeds the maximum range of the apparatus.

It will be apparent. that except for displacement in phase, the amplifier I05 will produce a curve having the same shape as the output curve of the amplifier I04. As shown in Fig. 13, the output wave shape I I6 of amplifier I04 simulates a secant curve and the output wave shape I I1 0! the amplifier I05 simulates a cosecant curve.

Elimination of ground reflection by electrical ground distance measurement In accordance with another embodiment of my invention, the distance to ground along the radio beam axis I3 is measured electrically. Such a system is represented schematically in Fig. 16. The structure of the scanner equipment illustrated is similar to that represented schematically in Fig. 3 except that the angle transmitters 26 and 3| may be omitted; likewise the gyro system 53 is omitted. It will be understood, however, that my invention is not limited to the type of scan or to the type of indication, which has been described for the sake of illustration.

A moving gate is applied to a suitable portion of the target-responsive indicating apparatus including the receiver 4| and the indicator 42. For example, it may be applied to a channel in the receiver 4I or to the indicator 4!, as in the arrangement of Fig. 3. For supplying such a moving gate to moving gate terminals 41, a moving gate generator II8 is provided, which is responsive to target range measurements obtained from the receiver 4| through a connection II9. In effect, the apparatus of Fig. 16 utilizes the range 17 measurements produced by the receiver 4I whenever the radio beam I3 strikes the ground, in order to produce a moving gate having a length corresponding to the range measurement which, in this case, is the distance to ground.

As illustrated in Fig. 1'1, where the curve I2I represents the maximum scanning angle or cone of the radiator II with a counter-clockwise direction of spin and with the maximum radial range of the apparatus, as soon as the radio beam has been turned sufilciently low a beam as long as the maximum efl'ective range strikes the ground at a point A and the receiver 4I produces a range measurementro. As the radiator II continues to rotate, the measured range becomes progressively shorter, as represented by the radii I, 2, 3, etc., up to 1'", which is the minimum value corresponding to the position of the radiator II with the beam I3 in its lowermost position. As the radiator II continues to spin, the ground range increases again until the beam strikes the point B, after which a beam as long as the maximum effective range no longer strikes the ground, and there is no further need for eliminating ground reflection. It will be understood that the radius To of the circle I2I represents the maximum range R. of the apparatus.

For producing a moving gate continuously proportional to the varying values of 1', measured by the receiver 4|, a suitable circuit such as a vacuum tube circuit may be employed. Such a circuit is illustrated in Fig. 18.

Ground distance measurement circuit As illustrated in Fig. 18. the moving gate generator II8 as a whole comprises a square wave generator I22 adjustable with regard to the length of the wave produced thereby, a range gate generator I23, means for coupling the range gate generator I23 to the square wave generator I22, which coupling means may take the form of a bufier or cathode follower stage I24, for example, a secon .range gate generator I25 synchronized with th output of the first range gate generator I23, a pair of coincidence devices I26 and I21, a pair of low time constant peak voltmeters I28 and I29, a pair of integrators I3I and I32, a gate-width limiting diode I33, a bias adjusting device I34 with a feed-back connection I35 to the square wave generator I22. and a constant current device I36 associated with the bias adjusting device I34.

The adjustable square wave generator I22 may take the form of a single-pulse multivibrator circuit comprising a pair of electric valves, such as triode vacuum tubes I31 and I38, for example. The tubes I31 and I38 have conventional load resistors I39 and I4I,'respectively, and have a common cathode-resistor I42 for coupling the tube I31 to the tube I38. For coupling the tube I38 to the tube I31, resistance-capacity coupling is employed comprising a coupling condenser I43 1 and a grid leak resistor I44 connected between the control grid of the tube I38 and the positive terminal of a source of anode voltage supply I45 for positively biasing the tube I38.

The initial tube I31 has a control electrode or grid I46, capacity coupled to the trigger-pulse input terminals 5|, and also connected through a grid resistor I41 to the conductor I35 providing a bias potential.

The square wave generator I22 is designed to have the output wave appear at an output terminal I48 which is electrically connected to the anode of the tube I36. The output terminal I48 control connections or moving gate terminals 41.

The cathode-follower stage I24 may take the form of a triode vacuum tube having a control grid coupled to the output terminal I48 of the square wave generator I22 through a coupling condenser I43 and a grid resistor I5l.

The cathode-follower stage I24 includes a cathode resistor I52 with a coupling by means of a peaking condenser I53 to the first range gate generator I23 for synchronizing the latter with the termination or descending potential portion of the square wave appearing at the square wave output terminal I48. The capacity of the condenser I53 is made relatively small in ordmto produce the peaking action.

The range gate generator I23 may take the form of a blocking oscillator comprising an electric valve such as a triode vacuum tube I 54, for example, having a pair of transformer primary windings I55 and I56 each connected in series with the anode lead to the anode supply source I45. In inductive relation to the transformer winding I55 is a transformer winding I51 connected at one end to the negative terminal of the supply source I45 through a grid resistor I58 and connected at the other end through a condenser I59.- A junction terminal I6I of the winding I51 and the resistor I58 is connected to the control grid I62 of the triode I54.

In inductive relation to the transformer primary winding I56 is a secondary winding I63 across which a potentiometer resistor I64 may be connected. Associated with the potentiometer resistor I64 is a movable tap I65 for providing an adjustable amplitude output voltage which will be in the form of a square wave by virtue of the known characteristics of the blocking oscillator circuit I23.

The second range gate generator I25 is coupled to the first range gate generator I23 through a difierentiating or peaking circuit including a condenser I66 for synchronizing the generator I25 with the end of the output wave of the generator I23.

The second range gate generator I25 may also be in the form of a blocking oscillator having elements and connections which are similar to those described in connection with the range gate generator I23. A potentiometer including a movable brush I61 is provided for supplying an adjustable amplitude square wave output.

The coincidence devices I26 and I21 are double input electric control devices and may take the form of vacuum tubes, so arranged as to have two input or control connections each. Thus each tube should be at least a tetrode with a pair of control electrodes or grids. As shown, however, the tubes I26 and I21 are in the form of pentagrid converters having conventional pairs of shield grids and conventional suppressor grids. The tube I26 has a first control electrode or grid I68 coupled in any suitable manner, as by means of a coupling condenser I69 and a grid leak resistor I1I, to the output terminal I 61 of the second range gate generator I 25. In a similar manner, the tube I21 has a first control electrode or grid I12 resistance-capacity coupled to the output terminal I65 of the first range gate generator I23. The tubes I26 and I21 also have second control electrodes or signal grids i 13 and I14, respectively, separately resistance-capacity coupled to an input connection II9 from the receiver 4| (Fig. 16). The connection III! is the one at which the receiver 4I supplies what is 19 customarily known as the video ouput, which will be the ground signal in case the radio beam axis I3 is pointed toward the ground.

The supplying anode-cathode current to the tubes I26 and I21 in parallel a source of unid1- rectional voltage I may be provided. Pulse transformers are provided with primary windings I16 and I11 in series with the anode connections of the tubes I26 and I21, respectively. In Inductive relation to the transformer windings I16 and I11 are secondary windings I18 and I19, respectively, and for adjustment of the magnitude of the pulse output, potentiometers having output taps I8I and I82 are provided. The secondary windings I18 and I19 are connected to give output voltage of polarity opposite to the anodes of the tubes I26 and I21.

The tubes I26 and I21 are preferably of a type such as those sold as GSA? tubes in which driving either grid I68 or I13 (in thecase of the tube I26) below a cut-off potential extinguishes current irrespective of the potential of the other grid. It will be understood that the shield grids are maintained at a substantially fixed potential and that the suppressor grid of each tube is maintained at cathode or ground potential.

The low time constant peak voltmeters I28 and I29 may be in the form of electric valves such as triode vacuum tubes having their control grids resistance-capacity coupled to the output terminals I8I and I82 of the coincidence devices I26 and I21, respectively. In order to cause the tubes I28 and I29 to function as peak voltmeters or pulse wideners to spread the wave, condensers I83 and I84, respectively, are connected between the cathodes of the tubes I28 and I29 and the negative terminal of the supply source I15. For reducing the time constant of the peak voltmeters to a relatively low value, resistors I85 and I86 are shunted across the condensers I83 and I84, respectively, The time constants of the circuits I83, I85 and I84, I86 are so chosen that the condensers I83 and I84 discharge within a time duration of the order of magnitude of the pulse repetition rate of the pulse system with which the apparatus is employed.

For establishing a biasing potential for the square wave generator I22 which will depart from I the average value in accordance with the relative magnitudes of the voltages measured by the devices I28 and I29, the integrators I3I and I32 are provided and they are connected in series. They may take the form of electric control devices or electric valves such as triode vacuum tubes having control electrodes or grids resistance capacity coupled to the cathodes or output connections of the peak voltmeter tubes I28 and I29, respectively. The tube I32 is negatively biased and the tube I3I is adjustably biased by means of a potentiometer tap I10 cooperating with a series resistor I80 connected between the devices I34 and I36.

The junction terminal I81 of the integrator tubes I3I and I32 serves as the bias control potential output for the integrator circuit.

For limiting the potential output and thereby limiting the gate width of the circuit of Fig. 18, the gate width limiter I33 is connected between the integrator output terminal I81 and a voltage divider terminal I88. The voltage divider terminal I88 may be the junction terminal of a pair of resistors I89 and I9I connected in series to the pply source I15. For avoiding abrupt variations in the potential of the terminal I89,

a by-pass condenser I92 may be connected across the resistor I9I. The gate limiter I33 may take the form of an unsymmetrical current conducting device, rectifier, diode vacuum tube, or the like. The limiter I33 is connected with its cathode at the potential-divider terminal I88 and its anode at the integrator terminal I31.

Although the devices I3I, I32 have been referred to as integrators for the sake of identification, it will be understood that in order to obtain fast action'of the circuit, the arrangement is preferably such that excessive smoothing action or integration is not obtained. To this end the circuit constants are so selected that the capacity loading on the integrators I3I, I32 is not so great as to slow down the action of the circuit. Distributed capacity may be relied upon 'to provide the requisite degree of integration.

If a separate condenser I is employed its capacity should be relativelysmall.

The circuit I3II32 may be thought of as a mixer for providing a single-ended output from two inputs rather than as an integrator inasmuch as little integration is desired.

For supplying the potential of the integrator output terminal I81 to the bias connection I35 of the square wave generator I22, without reaction between the stages of the circuit, the cathode follower I34 may be interposed between the terminal I81 and the connection I35. The cathode follower I34 is shown as comprising a triode vacuum tube with a control electrode directly connected to the integrator output terminal I81.

For causing the circuit to drift to a maximum gate indication in case no input signals are received from the receiver 4I through the connection I I9, a relatively large resistor or bleeder I93 is connected between the control electrode and the anode of the tube I34.

For supplying a control grid bias to the integrator tube I3I, and maintaining the bias constant with respect to the potential of the point I81, the constant current device I36 is connected in series with the cathode lead of the tube I34 and for adjustment of bias, as previously explained, the potentiometer I10, I80 is connected in series between the tubes I34 and I36. The tube I36 may take the form of a pentode having a conventional suppressor grid tied to ground and having a screen grid I94 connected through a dropping resistor I95 to the positive terminal of the supply source I15. However, the control grid I96 of the tube I36 is permanently tied to ground or the negative terminal of the supply source I15, causing the tube I36 toconduct a substantially constant current. A cathode resistor or stabilizer may be provided.

Operation w th electrical ground distance measurement The principle of operation of the circuit of Fig. 18 and of the apparatus of Fig. 16 as a whole is illustrated by the graphs of Fig. 19. It will be understood that in radio locator systems of the type to which reference is made herein, the transmitter (not illustrated) and the receiver M are synchronized in action by means of a triggering pulse which is ordinarily generated by a generator associated with the transmitter. Such a triggering pulse is represented by the peaked wave I98 in Fig. 19A; Such pulses are repeated at a suitable repetition rate which may equal 2,000 per second, e. g., in certain types of radio locator apparatus. In response to each such trigger pulse I98, a pulse I99 as shown in Fig. 198 is produced by the transmitter. The pulse I99 is actually in the form ofa wave train of high frequency microwave oscillations, for example, a train of 3000 such oscillations in the case of centimeter waves each continuing for a pulse duration of one micro-second. The pulse wave form I99, therefore, actually represents the rectified envelope of such a transmitted pulse consisting of microwave oscillations directed in a beam by the radiator I I along the beam axis I3.

In such radio locator apparatus known means have also been provided for producing a so-called wide gate, which is a square wave of relatively long duration, as illustrated at MI in Fig. 190. The means for providing such a wide gate are so designed that the gate begins after the termination of the transmitted pulse and terminates well before the production of a succeeding transmitted pulse. The gate I is supplied to the receiver 4I through the Wide gate terminals 45 in order to render the receiver 4| responsive only during the time period between transmitted pulses so that the receiver can pick up only reflected pulses and will not be responsive to direct transmission of energy from the transmitted pulse I99. Such receivers are customarily provided with a second gate input terminal referred to as a narrow gate connection for further limiting the time duration during which the receiver is responsive a in order to cause the apparatus to pick-up reflections from only a Particular target region, for example, when so desired. In place of using such a narrow gate connection in the customary manner, I may supply a moving gate to such terminals or I may supply a moving gate to suitable control connections 41 of the indicator 42 as described in connection with the apparatus of Fig. 3.

Assuming the latter arrangement, a moving gate having the form illustrated at 202 in Fig. 19D (corresponding to Fig. 7) is produced by the moving gate generator II8 (Fig. 16), and supplied to the control connection 41 of indicator 42. Since the moving gate generator H8 is synchronized through the trigger input connection 5|, which directly or indirectly synchronizes also the wide gate 20I of the receiver 4|, the moving gate 202 may be arranged to commence at the same instant as the wide gate 20I of the receiver.

Referring to Fig. 18, the moving gate is formed by the multivibrator I22 in the manner described in connection with Fig. 6. The gate length is determined by the bias fed back through the conductor I35, which corresponds to the connection 52 of Fig. 6. The termination of the moving gate 202 along the vertical descending line 9| (Fig.1

19D) trips the range gate generator I23 and causes a relatively short time duration square wave or range gate 203 to be generated (Fig. 19E) The time duration of the square wave 203 is fixed, being determined by the circuit constants of the generator I23. In a similar manner, the termination of the first range gate 203 trips the second range gate generator I25 and causes the commencement of a second range gate 204 (Fig. 19F). The range gates 203 and 204 are displaced along the time axis but are substantially contiguous, i. e., one begins approximately when the other ends.

A signal received by the receiver M and supplied to the moving gate generator II8 through connections II9 has a wave form 205, as illustrated in Fig. 19G. For reasons which will be explained hereinafter, the form 205, which is a ground signal when the receiver is receiving reand 204.

Assuming that a variation has taken place in the distance to ground causing the ground signal 205 to occur earlier or later than the assumed time of occurrence, all or a preponderant portion of the ground signal 205 will occur during the time interval of one of the range gates 203 or 204. Assuming, for example, that distance along the radio beam I3 to ground is decreasing, a preponderant portion of the ground signal 205 will take place during the time period of the range gate 203. In this case, the coincidence device I21 (Fig. 18) will have both of its control grids I12 and I 14 energized simultaneously, the grid I14 being energized by the ground signal received from the connection H9, and the grid I12 being energized by the output wave 203 of the pulse generator I23.

A relatively strong output pulse 206 will there'- fore be supplied to the peak voltmeter I29. The

, other coincidence devi'ce I26, however, will not be caused to carry current, or will carry only rela-- tively little current, for the reason that although a ground signal is supplied to the screen grid I13 from the receiver connection N9 the control grid I68 will either not be energized at all during the continuance of the ground signal 205, or be energized for only a minute final fraction of the time duration of the ground signal 205. Consequently, the integrator tube I32 will have a greater input than the integrator tube I3I, causing its impedance to fall thereby lowering the potcntial of the integrator output terminal I81. This in turn causes the cathode follower I34 to increase its impedance, and to reduce the potential of the feedback connection I35, since current is held constant by the device I36 and its impedance falls to hold the current constant. This fall in :potentia1 lowers the bias of the square wave generator I22, causing the length of the output wave to be reduced.

Inasmuch as the peak voltmeters I28 and I29 have sloping output wave forms, as illustrated in Fig. 191, a smooth variation in the integrator output is obtained with variations in the portion of the ground signal 205 occurring during the time durations of the first and second rangegates 203 and 204. The signal fed back through the connection I35 is progressively varied in response to successive transmitted pulses and corresponding successive reflected ground signals so as to adjust the time duration of the moving gate 202 and to cause the ground signal 205 to be split.

with respect to the time of occurrence between the range gates 203 and 204. v

Variations in the distance to ground along the radio beam axis l3 take place continuously whenever the beam is pointed toward the ground. Theseeare cyclical variations due to the spinning action of the scanner as illustrated by Fig. 17. The nod action also causes cyclical variation. Furthermore, if the aircraft pitches or if it changes in altitude, variations result in radial distance to ground. The latter variations are not cyclical but are alsocontinuous or progressive. The movin gate circuit of Fig. 18 continuously follows all such variations and eliminates the ground signal from the screen 2I of the indicator 42. Referring to Fig. 17, it will be seen that the gate length of the moving gate must be a maximum when the beam strikes the ground at the point ,A, must then decrease progressively to 23 represent decreasing distance to ground until the point E is reached, and then must increase to a maximum at the point B. During the time interval required for the beam to rotate from the point B counter-clockwise back to the point A, the gate length should remain a maximum. The drift resistor I 93 (Fig. 18) insures that the apparatus will remain at maximum range in readiness for the moment when the beam strikes the ground at point A. The variations referred to are progressive and therefore the gate 202 follows progressively to control the indicator 42. However, if a target should be located, the signals reflected therefrom will represent a discontinuity and will not affect the auto ranging apparatus of Fig. 18

or the moving gate 202; The automatic reduction 7 of range to less than slant distance to ground along the beam axis takes place regardless of the type of scan employed.

It will be understood that the connection [I9 from the receiver must be taken from a point in the receiver circuits ahead of the point at which a moving gate control voltage is applied in order to leave the appropriate channel of the receiver 4| active long enough to produce the ground signal 205. It will be seen from Fig. 19, however, that the moving gate 202 is terminated before the occurrence of the ground signal 205. Consequently, the ground signal does not appear in the portions of the receiver following the moving gate connection, and does not appear in the indicator 42.

If exceedingly rapid variations in the rate of change of distance to ground are anticipated it may be desirable to provide means to make the gain of the auto ranging circuit become greater with larger error, that is, larger deviation between the position of the ground signal 205, referring to Fig. 19G, and the position of the common sides of the range gates 203 and 204. This may be accomplished by reforming the ground signal 205 so as to form a relatively triangular wave shape, shown by dotted lines 208. Conversely the same results may be accomplished by modifying the form of the range gates 203 and 204 by rounding ofic the negative discontinuity of gate 203 along the dotted line 209 and similarly rounding the positive discontinuity of the range gate 204 along the dotted line 2, as shown in the Figs. 19E and 19F. Suitable circuits for effecting such modification in the shape of the range gates may be employed. Although I may modify the arrangement Of the range gate generators I23 and I25 to effect the desired change in shape of the range gates, for the sake of simplicity in the drawing, I have indicated the means for effecting the change in range gates schematically by means of rectangles 2 I2 and 2l3 (Fig. 18) representing suitable wave shaping circuits.

Runway approach control The elimination of ground or surface reflections in accordance with my invention is valuable for increasing the effectiveness of object location regardless of the purpose for which the object location equipment is primarily used. For example, it is valuable for increasing the safety of aviation by making object location equipment responsive to the presence of other aircraft or responsive to stationary obstacles jutting upward from the ground. The elimination of ground refiections is particularly valuable, however, for certain purposes such as traflic control and runway approach control because in crowded airports, a large number of planes may desire to land at substantially the same time during a fog or other conditions when visibility is low and the effective because the operator in an airport control center is aware only of the presence of airplanes which have communicated with the operator; whereas the pilot of an airplane equipped with radio object location apparatus, having the ground reflections eliminated in accordance with my invention, has an indication upon his screen not only of the presence of other aircraft but also of their exactorientation. Accordingly, in accordance with my invention, it is unnecessary for airplanes to land one at a time awaiting instructions from an airport control center and pilots may land in rapid succession after assuring themselves of the absence of aircraft or other obstacles in the intended line of approach to the airfield. For example, referring to Fig. 2, if the pilot of the airplane I8 desires to make a landing at a landing field located at the point 20, the pilot knows that such a landing can safely be made if another aircraft is located at the point 23 but not if another aircraft is located below him at the point 93.

The elimination of ground reflections makes it possible for the pilot of the airplane I8 to obtain an indication of an obstacle at the point 93 without obliteration of the indication by the ground interference 26 (Fig. 1). v

The elimination of ground reflections from the screen 2| also enablesa pilot when approaching an airfield to descend along a more gradual slope, that is, to reduce altitude a greater distance from the landing field than would otherwise be possible, in view of the danger of striking tall chimneys, water towers and the like, which may be relatively close to the landing field and not visible during a fog. Since the radio beam l3 encounters a discontinuity when it is swept past such a sharp obstruction as a tall chimney or water tower, the automatic range apparatus of Figs. 16 and 18 will not interfere with the indication of such an obstruction on the screen 2I. Nevertheless the gradually changing measured distance to ground or continuous range variation resulting from the radio beam I3 sweeping along the ground, when it is pointed toward the ground,

- will maintain the effectiveness of the auto ran ing equipment for eliminating ground or surface reflections. This will be true even in the case of rolling ground and areas with gradually varying slop-es whereas sharp obstructions such as cliffs, edges of canyons and upwardly protrudin obstacles such as towers will represent discontinuities and produce indications on the screen 2 I.

The characteristics of my ground-reflectionelimination apparatus make obstacles stand out more clearly on the screen 2| and prevent relatively smooth or gently rolling ground repre-- senting a good landing surface from producing obliterating indications.

As many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Whatisclaimedis:

1. A radio object locator operated at a point above ground and unaifected by ground reflections, comprising a spiral sweep scanner having a radio beam axis rotatable about a spin axis to describe a cone and pivoted on a nod axis transverse thereto and rotatable with respect to the ,spin axis to change the apex angle of the cone,

object signal responsive indicating apparatus with an input coupling and a control connection for rendering the apparatus responsive only when the connection is energized, radio means operatively coupled to said scanner for applying to the input coupling of said indicating apparatus signals representing objects in the beam of said scanner, apparatus for measuring the distance to ground along said radio beam axis, and means interposed between said distance measuring apparatus and said control connection for converting the distance measurement into an indicator control quantity energizing said control connection and having a time du ation shorter than that required for signals to be received over the distance measured by said radius measuring apparatus.

2. A radio object locator for operation at a point above ground with freedom from interfer ence due to ground reflections, comprisin a movable radio beam scanner for sweeping a radio,

for measuring the distance to ground along said radio beam axis, and mechanism interposed between said measuring means and the control connection of the reflected energy responsive apparatus for maintaining the eflective range of the apparatus shorter than the measured radial distance to ground.

3. An object locator ,for operation at a point above ground, comprising a radio scanner for continuously sweeping a radio beam through space in which an object or obstacle is to be located, apparatus responsive to radio energy reflected from the object having an input coupling from said scanner, means for measuring the distance to ground along said beam, and means for automatically varying the effective range of said apparatus in accordance with said measurement.

4. In combination, a variable-range continuously variable-directivity radio object locator operating at a point above ground for locating objects in a beam between said locator and ground, and means for automatically reducing the range thereof in accordance with the distance to ground along said beam synchronously with the directivity variation thereof.

5. A radio object locator for operation at a point above ground with freedom from interference due to ground reflections, comprising a spiral sweep scanner having a radio beam axis rotatable about a spin axis to describe a cone and pivotedon a nod axis transverse to and rotatable with respect tothe spin axis to change the apex angle of the cone, a radio receiver with an input connection from said scanner, an indicator with an input connection from said re-- ceiver, one of the pair of devices designated as the receiver and the indicator having a control connection rendering the device responsive only when the control connection is energized, altimee ter means for producing an altitude-responsive signal, a computer responsive to said altituderesponsive signal and to the radio beam orienta- 'ceived by said scanner over a radial distance computed by saidcomputer, said computer being arranged to solve the equation h sin cos 0' where 1- is the distance to ground along a radio beam axis of the scanner, h is the altitude, 1/ is the nod angle of the scanner, and 0 is the spin angle.

6. A radio objectlocator for operation at a point above ground with freedom from inter1'er-' ence due to ground reflectidns, comprising a spiral sweep scanner having a radio beam axis rotatable about a spin axis to describe a cone and pivoted on a nod axis transverse to and rotatable with respect to the spin axis to change the apex angle of the cone, indicating apparatus responsive to radio energy reflected from the object having an input coupling from said scan- H ner and a control connection rendering the apparatus responsive to objects only when the connection is energized, a computer arranged to solve for the radial distance to ground along said radio beam axis, input connections to said computer, means for supplying thereto indications of scanner spin angle and nod angle, and means for supplying the output of said computer to the control connection of said reflected energyresponsive indicating apparatus for maintaining said apparatus responsive for a time interval less than that required for si nals to be received by said scanner from a radial distance computed by said computer.

7. A radio object locator for operation at a point above ground with freedom from interference due to ground reflections, said locator comprising a spiral sweep scanner having a radio beam axis rotatable about a spin axis to describe a cone and pivoted on a nod axis fixed to and rotatable with respect to the spin axis to change the apex angle of the cone, indicating apparatus responsive to radio energy reflected from the object having an input coupling from said scanner and a control connection for controlling the eifective distance range of the apparatus, a computer arranged to solve for the radial distance to ground along said radio beam axis, said computer having a plurality of input connections and an output connection, means for transmitting thereto indications of scanner spin angle and scanner nod angle, and means interposed between -said output connection and the control connection of said reflected energy-responsive apparatus for converting the output of said computer into a control quantity proportional to computed distance to ground along said radio beam axis.

8. A radio object locator for operation at a point above ground with freedom from interference due to ground reflections, said locator comprising a movable" scanner having a radio beam axis and means arranged to sweep said axis 

